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        <h3 id="一-运行时的数据区域"><a href="#一-运行时的数据区域" class="headerlink" title="一.运行时的数据区域"></a>一.运行时的数据区域</h3><p><img src="/blogimg/jvm/1.png" alt=""></p>
<h4 id="（1）程序计数器"><a href="#（1）程序计数器" class="headerlink" title="（1）程序计数器"></a>（1）程序计数器</h4><p>程序计数器(Program Counter Register)是一块较小的内存空间,它可以看作是当前线程所执行的字节码的<strong>行号指示器</strong>—–编译原理中指向下一条指令的位置</p>
<p>Java虚拟机的多线程是通过线程<strong>轮流切换并分配处理器执行时间</strong>的方式来实现的，每条线程都需要有一个<strong>独立的程序计数器</strong>,各条线程之间计数器<strong>互不影响</strong>,独立存储,我们称这类内存区域为“线程私有”的内存</p>
<p><strong>注意：</strong></p>
<ul>
<li>如果线程正在执行的是一个Java方法,这个计数器记录的是正在执行的虚拟机字节码指令的地址</li>
<li>如果正在执行的是Native方法,这个计数器值则为空(Undefined)</li>
</ul>
<h4 id="（2）Java虚拟机栈"><a href="#（2）Java虚拟机栈" class="headerlink" title="（2）Java虚拟机栈"></a>（2）Java虚拟机栈</h4><p>Java虚拟机栈(Java Virtual Machine Stacks)也是线程私有的,它的<strong>生命周期与线程相同</strong>。</p>
<p>虚拟机栈描述的是Java方法执行的内存模型:<strong>每个方法在执行的同时都会创建一个栈帧</strong>(Stack Frame )用于<strong>存储局部变量表</strong>、操作数栈、动态链接、方法出口等信息。每一个方法从调用直至执行完成的过程,就对应着一个栈帧在虚拟机栈中入栈到出栈的过程。</p>
<p>局部变量表存放了编译期可知的各种基本数据类型、对象引用（<strong>reference字段</strong>）和了一条字节码指令的地址，方法的开始</p>
<p>一个重要的异常 <strong>stackOverflowError  当内存不够的时候抛出 OutofMenoryError</strong></p>
<h4 id="（3）本地方法栈区"><a href="#（3）本地方法栈区" class="headerlink" title="（3）本地方法栈区"></a>（3）本地方法栈区</h4><p>和虚拟机栈相似值不过使用的是native的方法服务</p>
<blockquote>
<p>栈区大小使用 -Xss进行配置     </p>
</blockquote>
<blockquote>
<p>引申：参数 -XX：PermSize  -XX ：MaxPermSize  — 指定非堆内存的最大容量</p>
</blockquote>
<h4 id="（4）java堆"><a href="#（4）java堆" class="headerlink" title="（4）java堆"></a>（4）java堆</h4><p>Java堆(Java Heap)是Java虚拟机所管理的内存中最大的一块。Java堆是<strong>被所有线程共享的一块内存区域,在虚拟机启动时创建，用来存放对象实例</strong></p>
<p>指定java堆大小的参数 -Xms  -Xmx  –最大值和最小值</p>
<h4 id="（5）方法区"><a href="#（5）方法区" class="headerlink" title="（5）方法区"></a>（5）方法区</h4><p>所有<strong>线程共享，存储常量静态常量，编译后的各种代码</strong>等数据</p>
<blockquote>
<p>运行时常量池-方法区的一部分用来储存java的class类的各种其他信息</p>
</blockquote>
<h4 id="（6）额外部分-直接内存使用’"><a href="#（6）额外部分-直接内存使用’" class="headerlink" title="（6）额外部分 直接内存使用’"></a>（6）额外部分 直接内存使用’</h4><p>当java使用NIO的时候，直接使用外部内存而不是使用 虚拟机的虚拟内存</p>

      
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              <div class="post-toc-content"><ol class="nav"><li class="nav-item nav-level-3"><a class="nav-link" href="#一-运行时的数据区域"><span class="nav-number">1.</span> <span class="nav-text">一.运行时的数据区域</span></a><ol class="nav-child"><li class="nav-item nav-level-4"><a class="nav-link" href="#（1）程序计数器"><span class="nav-number">1.1.</span> <span class="nav-text">（1）程序计数器</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#（2）Java虚拟机栈"><span class="nav-number">1.2.</span> <span class="nav-text">（2）Java虚拟机栈</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#（3）本地方法栈区"><span class="nav-number">1.3.</span> <span class="nav-text">（3）本地方法栈区</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#（4）java堆"><span class="nav-number">1.4.</span> <span class="nav-text">（4）java堆</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#（5）方法区"><span class="nav-number">1.5.</span> <span class="nav-text">（5）方法区</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#（6）额外部分-直接内存使用’"><span class="nav-number">1.6.</span> <span class="nav-text">（6）额外部分 直接内存使用’</span></a></li></ol></li></ol></div>
            

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